1. Field of the Invention
This invention relates generally to a pedicle screw distraction device that causes distraction between opposing vertebrae and, more particularly, to a pedicle screw distraction device that causes distraction between opposing vertebrae, where the device includes a connector link having a locking plate for locking pedicle screw heads to the link and an angled end that provides a desired angle between the pedicle screws.
2. Discussion of the Related Art
The human spine includes a series of vertebrae interconnected by connective tissue referred to as intervertebral discs that act as a cushion between the vertebrae. The discs allow for movement of the vertebrae so that the spine can bend and rotate.
The intervertebral disc is an active organ in which the normal and pathologic anatomies are well known, but the normal and pathologic physiologies have not been greatly understood. The intervertebral disc permits rhythmic motions required of all vertebrate animals in their various forms of locomotion. The disc is a high-pressure system composed primarily of absorbed water, an outer multilayered circumferential annulus of strong, flexible, but essentially inelastic collagen fibers, and an inner core of a hydrogel called the nucleus pulposus. The swelling of the contained hydrogel creates the high pressure that tightens the annular fibers and its laminations. Degeneration of discs in humans is typically a slow, complex process involving essentially all of the mechanical and physiologic components with loss of water holding capacity of the disc. Discogenic pain arises from either component, but is primarily due to altered chemistry. When this pain is severely disabling and refractory to non-operative, the preferred contemporary treatments are primarily surgical, particularly fusion and/or disc replacement.
Annular collagen fibers are arranged in circumferential belts or laminations inserting strongly and tangentially in right- and left-handed angulated patches into each adjacent vertebral body. Inside the annular ring is contained an aggrecan, glycosaminoglycan, a protein-sugar complex gel having great hygroscopic ability to hold water. The swelling pressure of this gel of the nucleus maintains the pressure within the annulus, forcing the vertebrae apart and tightening the annular fibers. This tightening provides the primary mechanical stability and flexibility of each disc of the spinal column. Further, the angulated arrangement of the fibers also controls the segmental stability and flexibility of the motion segment. Therefore, the motion of each segment relates directly to the swelling capacity of the gel and secondarily to the tightness of intact annulus fibers. The same gel is also found in thin layers separating the annular laminar construction, providing some apparent elasticity and separating the laminations, reducing interlaminar torsional abrasion. With aging or degeneration, nucleus gel declines by desiccation or the loss of water, while collagen content, including fibrosis, relatively increases.
Disc degeneration, which involves matrix, collagen and aggrecan, usually begins with annular tears or alterations in the endplate nutritional pathways by mechanical or patho-physiological means. However, the disc ultimately fails for cellular reasons. As a person ages, the discs in the spine go through a degenerative process that involves the gradual loss of the water holding capacity of the disc, referred to as desiccation. As a result of this loss of water, the disc space height may partially or completely collapse, which may lead to chronic back pain disorders and/or leg pain as a result of the nerves being pinched.
Progressive injury and aging of the disc occurs normally in later life and abnormally after trauma or metabolic changes. In addition to the chemical effects on the free nerve endings as a source of discogenic pain, other degenerative factors may occur. Free nerve endings in the annular fibers may be stimulated or irritated by stretching as the disc degenerates, bulges, and circumferential delamination of annular fibers occurs. This condition may lead to a number of problems. It has been shown that a person's disc is typically taller in the morning when a person awakes. This phenomenon may be due in part to the reduction of body weight forces on the disc when lying in a recumbent position overnight that causes the disc height to restore and provide rehydration. Therefore, the reduction of compressive forces on the disc may help to restore disc height and function.
As discussed above, as a person ages, the discs of the spine degenerate, and the disc space height collapses. Further, the ligaments and facets of the spine degenerate as well due to increased forces and stresses on these structures that occur as the disc loses its weight-bearing capacity. These problems can lead to a reduction in the foraminal height of the vertebrae. The neuro-foramen is an opening through the vertebrae that allows the nerve root of the spine to pass through. Because the nerve passes through the foramen, the nerve will often get pinched as the disc height collapses, leading to various types of back pain. Further, these problems often lead to difficulty in walking. Additionally, lateral and central canal stenosis can occur as a result of disc degeneration and collapse further causing the nerve roots to get pinched in the spinal canal. These conditions can lead to a condition referred to neurogenic claudication, where a patient can only walk a short distance before experiencing considerable back and leg pain. Typically, patients respond by sitting down and bending forward resulting in flexing of the spine and reducing pressure on the nerves relieving their symptoms.
The treatment of degenerative disc disease and associated spine ailments is one of the most costly medical conditions with an estimated annual direct cost in the United States of thirty-three billion dollars and a total annual societal cost exceeding one hundred billion dollars. Indeed, in one's lifetime most individuals will experience an episode of significant back and/or neck pain.